Signals often varied depending on the country or railway company who used them, but in general a red flag or light meant stop, green meant go and amber to slow down.
In addition, in the USA, moving the lamp up and down vertically was a signal to proceed; moving them across the track was a signal to stop; moving them in a circle was a signal to back up; or holding the lamp out steadily was a signal to reduce speed and take caution.
In Canada, in the past, since passengers were a rarity at small town stations, the train would not stop unless it was flagged down. When passengers wanted the train to stop they used a green and white lantern side-by-side.